Induction furnace control

ABSTRACT

An induction furnace is disclosed. The furnace has a hearth with sidewalls and a floor, and an electrical induction heater in communication with the hearth through the floor body means of a throat. In use metal contained in the hearth is heated to form a metal bath. The furnace is provided with means to control superheating of the metal bath by maintaining regions of the metal of the bath adjacent the sidewalls at least partly solidified.

FIELD OF THE INVENTION

This invention relates to channel type induction furnaces used in theprocessing of metal and specifically, but not limited to, the control ofsuch processes.

BACKGROUND TO THE INVENTION

Induction furnaces are used to process metals by the melting andsmelting thereof. An induction furnace normally has a refractory linedcontainment area, called the hearth, with an electrical induction heaterlocated in the floor of the hearth. A pool of liquid metal, called themetal bath, has to be maintained in the furnace to ensure continuousoperation. The liquid metal fills the induction heater to be heatedtherein. The floor, walls and pouring arrangement is normally designedto allow the metal to be drained from the induction heater byappropriate tilting of the furnace to facilitate replacement of theinductor and breaking out of the refractory materials in preparation forreplacement.

Feed materials are charged into the metal bath from the top or thesides. The feed material will initially penetrate the metal bath untilit comes to a rest on the hearth floor. Material charged subsequent tothis will rest on top of the previously charged feed material. Materialof lower density may float on the liquid surface.

The effect of the charging of relatively colder feed material is tolower the metal bath temperature. Some of the feed material will be incontact with the edges of the hearth. At the edges of the hearth, slagand frozen metal normally form a skull against the refractory lining,thereby providing a site onto which feed materials may be attached byway of some of the slag and metal from the bath freezing onto thelining.

A bridge of frozen and not yet reduced material can form over the metalbath through this mechanism. Such a bridge can grow with increasingamounts of cold feed material charged to the furnace. Eventually, thebridge may cover the whole of the metal bath surface and completelyseparate the liquid metal from the feed material above the bridge.

At this point, all of the metal under the bridge can be liquid and isprone to high degrees of superheating. Superheating occurs when ametal's temperature is increased to above its melting point. The degreeof superheating is defined as the difference between the meltingtemperature and the actual temperature of the bath.

The operators of the furnace might be unaware of the superheatingbecause of the physical barrier of the bridge in the furnace, causingthem to inadvertently allow further superheating of the metal. From theoperators point of view, the feed material does not appear to be meltingand the normal response to this is to increase the energy input to thefurnace.

The obvious danger of this is that the temperature of the metal bath canincrease to above the safe operating temperature of the refractorylining, causing the lining to be eroded and eventually to burn through.

Another danger is that gasses, such as oxygen and nitrogen, may dissolvein the superheated metal to a much higher degree than would be the caseif the metal temperature is maintained at lower levels.

If the refractory lining does not bum through, the metal bathtemperature will reach a point where the bridge is sufficiently weakenedto break under the weight of the feed material. This can cause a suddenpenetration of the metal bath by the feed material, resulting in asudden lowering of the metal bath temperature.

Cooling of the metal bath and especially solidification of the bathcauses a lowering of the solubility of gas in the liquid metal. Theeffect of this is that a large volume of gas is expelled from the metalbath, causing what is commonly known as a boiling action that isdangerous to people and equipment near the furnace. This is morepronounced if there are more gasses dissolved in the metal.

Practices have been developed to ensure that superheating does notoccur. These include constant temperature measuring of the metal bathand reducing the energy input when it becomes clear that superheating isoccurring. It is not always possible to obtain a temperature measurementof the metal bath because of the thickness of the bridge over the metalbath. Additionally, the effect of reducing the energy input to thefurnace is that the production rate of the furnace is lowered.

Practices have also been adjusted to ensure that a sufficient depth ofliquid metal is retained after liquid metal has been tapped from thefurnace and the addition of small amounts of feed materials to allow thecharged material to be melted before further material is fed to thefurnace. This means that feed materials that are charged, are rapidlysubmerged in the liquid bath.

This in turn means that feed material particles do not spend enough timeabove the liquid slag interface to be heated and potential surface areainto which heat energy, other than heat derived from electrical energy,can be transferred is therefore lost. The remaining surface area intowhich heat energy, derived from combustion of fuel, can be transferredis the liquid slag surface.

This also has the result that almost all of the metal in the furnace isliquid at any given time, meaning that any delay in feeding the furnacewith more feed materials will rapidly result in superheating of themetal bath. To overcome this, the energy input has to be reduced tocontrol the metal temperature and to prevent possible permanent damageto the refractory lining.

It is therefore very difficult to constantly maintain the operation ofan induction furnace at the point where the energy and feed materialinput is maximised and balanced to prevent either superheating of themetal bath or cooling of the furnace top and bridging of the chargedmaterial.

OBJECT OF THE INVENTION

It is an object of this invention to provide an induction furnace and amethod for operating an induction furnace which at least partlyalleviates the above-mentioned difficulties.

SUMMARY OF THE INVENTION

According to this invention there is provided an induction furnace witha hearth having sidewalls and a floor, an electrical induction heater incommunication with the hearth through the floor by means of a throat to,in use, heat metal contained in the hearth to form a metal bath,characterised in that the furnace is provided with means to controlsuperheating of the metal bath by maintaining regions of the metal bathadjacent the sidewalls at least partially solidified.

There is also provided for the means to control superheating to includemeans to substantially prevent charged feed material from contacting thesidewalls.

There is further provided for the superheat control means to includemeans to control the amount of slag in the furnace.

There is also provided for the means to control superheating of themetal bath to include maintaining an amount of feed material above themetal bath supported by a bridge of solidified metal or solidified slagor a combination of solidified metal and slag covering the metal bath,and for the amount of feed material to be sufficient to penetrate thebridge, as a result of weakening of the bridge through its heating,before the entire metal bath is melted.

The invention also provides for the furnace to have a shaft with anopening into the hearth through which the feed material is fed, theshaft to prevent feed material from contacting the sidewalls of thehearth.

A further feature of the invention provides for the metal bath depth,the shaft inner cross sectional area, and the distance between the shaftopening and the bridge, or any combination thereof, to be used tocontrol the frequency of feed material penetrating the bridge.

A still further feature of the invention provides for preheating of thefeed material to be done in the hearth and shaft, and for the preheatingto be done through the combustion of fuel.

A further feature of the invention provides for a plurality of feedtubes to extend between the shaft and the hearth, for the feed materialto be fed through the feed tubes to the hearth, and for the feedmaterial to be at least partially and indirectly preheated in the feedtubes by means of heat transfer through the sides of the feed tubes.

In accordance with this invention there is also provided for a method ofcontrolling superheating of the metal bath of an induction furnace asdefined above, by maintaining the metal bath at least partly solidifiedthrough the steps of providing an amount of feed material on asolidified metal bridge over the metal bath,

-   -   heating of the metal bath,    -   causing the solidification front to move away from the throat        thereby weakening the bridge and causing the feed material to        penetrate the metal bath through the bridge before the entire        metal bath is melted,    -   causing the metal bath temperature to be lowered and the        solidification front to be moved closer to the throat to        strengthen the bridge sufficiently to support subsequently fed        material.

A further feature of the invention provides for the method to includethe steps of removing slag from the furnace and preheating the feedmaterial.

A further feature of the invention provides for the method to includethe steps of using the metal bath depth, the shaft inner cross sectionalarea, and the distance between the shaft opening and the bridge, or anycombination thereof, to control the frequency of feed materialpenetrating the metal bath.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described below by way of exampleand with reference to the accompanying drawings in which:

FIG. 1 is a section through an induction furnace showing the state ofthe metal bath before penetration of the feed material.

FIG. 2 is a section through the same induction furnace after the feedmaterial has penetrated the metal bath.

FIG. 3 is a perspective view of a second embodiment of the invention.

FIG. 4 is a perspective view of the second embodiment of the inventionshowing the furnace without the shaft sidewalls and the roof.

DETAILED DESCRIPTION OF THE DRAWINGS

An induction furnace is generally indicated by numeral 1 in thedrawings. The furnace (1) has a hearth (2) with a floor (3), sidewalls(4) and a roof (5) within which a charging shaft (6) is formed. Thehearth (2) is lined with refractory material to contain the moltenmetal.

The furnace (1) is equipped with an induction heater (7) that is incommunication with the hearth (2) through a throat (8) that opens in thefloor (3). In use, the hearth (2) contains a metal bath (9) the metalbath (9) has an upper surface (10) that defines the upper extremity ofthe metal bath (9). The upper surface (10) can be covered with a slaglayer (not shown). The upper surface (10) may solidify partly or fullyto form a bridge (23) covering the metal bath (9). The metal bath (9)comprises liquid metal (11) and solidified metal (12) with asolidification front (13) between it.

The furnace (1) is used to melt and process scrap metal and previouslyreduced iron ore called DRI or direct reduced iron to produce liquidiron or steel. The furnace (1) is charged with feed material (14)through the shaft (6). The feed material can include scrap steel, DRIand the like, depending on the availability of feed materials and therequired product. The initial charge of feed material (15) will passthrough the shaft (6) to rest on the bridge (23) if the bridge (23) isstrong enough, or to penetrate the bridge (23) to rest on the floor (3).Additional feed material (16) charged will rest on top of the firstcharged feed material (15) in the shaft (6).

The bridge (23) can support material (15) fed to the furnace through theshaft (6) and can grow to cover the entire upper surface (10) of themetal bath as will be explained further on. It is possible for thebridge to grow to proportions that enables it to carry all the chargedfeed material (15, 16) without breaking or sagging.

The shaft (6) is positioned to charge the feed material (15) generallyin the area above the throat (8) and specifically not to charge any feedmaterial (15) in a position where it can touch a sidewall (4). Theamount of feed material (15) that actually rests on the bridge (23) isthat under the triangle (17) formed by the extended angles of repose(18) of feed material and the bridge (23). The feed material (16) abovethe triangle (17) is to a large degree supported by the shaft sidewalls(19).

If the initial charge of feed material (15) rests on top of the bridge(23), the metal bath (9) and the feed material (15) will be separated bythe bridge (23). Heating of the liquid metal (11) in the inductionheater (7) increases the temperature of the liquid metal (11) to a smalldegree but, more importantly, causes melting of the solidified metal(12). This causes the solidification front (13) to move away from thethroat (8) and changes the balance between the liquid metal (11) andsolidified metal (12). The relative amount of liquid metal (11) isincreased and the relative amount of solidified metal (12) decreased,although the total amount of metal in the metal bath (9) remains aboutthe same (ignoring the addition of a small amount of bridge (23) metalto the metal bath (9) as it is liquefied by the above heating.

The amount of liquid metal (11) will continue to increase and the amountof solidified metal will continue to decrease with continued heating.This could continue until the situation shown in FIG. 1 is reached,where the solidification front is further away from the throat (8) thanin FIG. 2, and the bridge (23) can no longer be supported by solidifiedmetal. As the amount of liquid metal (11) increases through heating, theheating weakens the bridge (23) and decreases the amount of liquid metal(11) under the bridge increases simultaneously with a decrease in theamount of solidified metal (12) supporting the bridge (23) therebyfurther weakening it.

The result of the weakening of the bridge (23) is that a point isreached where it cannot support the weight of the feed material (15)resting on it and collapses. The feed material (15) penetrates theliquid metal (11) to rest on the floor (3), or subsides to makes goodcontact with the liquid metal (11), and some of material (16) from theshaft (6) moves downward to join feed material (15). The design of thefurnace and the shaft is such that the point of penetration is reachedbefore the whole of the metal bath (9) is liquefied.

The feed material (15) temperature is lower than that of the liquidmetal (11) and therefore lowers the liquid metal (11) temperature withpenetration. This temperature lowering causes the balance between theliquid metal (11) and solidified metal (12) to change with a relativeincrease of solidified metal (12) and relative decrease of liquid metal(11). The solidification front (13) therefore moves toward the throat(8). This is the situation shown in FIG. 2 where the solidificationfront (13) is closer to the throat (8) than in FIG. 1. Obviously, thetotal amount of metal in the metal bath increases after penetration ofthe feed material (15), but this is balanced by tapping from the furnace(1).

Once the feed material (15) has penetrated the upper surface (10), thefeed material (16) above the triangle (17) will move down in the shaft(6). The bridge (23) will form again as a result of the cooling to be abarrier between the metal bath and the feed material (15) on top of it.The bridge (23) will be able to support the weight of the feed material(15) on top of it at this stage.

The process of heating of the liquid metal (11) will repeat, the amountof liquid metal will increase again with a concurrent decrease in theamount of solidified metal (12), and the solidification front (13) willbe moved away from the throat (8) again, and so on as described above.

The process therefore repeats for as long as sufficient feed material(15) remains in place on top of the meniscus (10) to force the collapseof bridge (23) before the whole of the metal bath (9) is liquefied, andthe energy input to the furnace (1) through the induction heater (7) ismaintained.

In case the initial charge of feed material penetrates the meniscus, theprocess will proceed from the point where the liquid metal (11)decreases relative to the solidified metal (12) and the solidificationfront (13) has moved towards the throat (8), as shown in FIG. 2.

For additional energy efficiency, gas burners (20) are installed in theroof (5) or side walls (4) to heat the feed material (15) above themeniscus (10) and to exit through the shaft (6) to heat the feedmaterial (16) located therein. This preheating of the feed material (15,16) is very efficient because the heated gas passes through the feedmaterial instead of over it, especially the feed material (16) locatedin the shaft (6). The preheating reduces the amount of electrical energyrequired to melt the feed material (15) after it penetrates the metalbath (9).

Additionally, the amount of solidified metal that will be liquefiedbefore the bridge (23) collapses can be reduced by the preheating of thefeed material (15). This is possible because preheating of the feedmaterial (15) will also heat the bridge (23) from above to weaken it.This further aids in ensuring that a portion of the solidified metal(12) always remains in place to protect the furnace (1).

It is clear from the above description that it is necessary to preventsignificant amounts of feed material touching the sidewall because itcould be attached to sidewall (4) and strengthen the bridge (23). Asstated before, such a strong bridge can support all the material (15)and (16) even if the whole of the metal bath (9) is liquefied. Withadditional heating, the liquid metal (11) will be superheated and createthe danger of a bum-through of the furnace (1).

Additionally, the formation of a strong bridge and subsequent highliquid metal temperature will allow the dissolution of greaterquantities of gas in the liquid metal. If the bridge is eventuallysufficiently softened by the heating to collapse, the rapid cooling andsolidification of the relatively large volume of liquid metal (11) dueto contact with the feed material (15) will cause a rapid decrease inthe solubility of gas in the liquid metal (11) and the expulsion of alarge volume of gas from the liquid metal (11), causing the metal bathto boil which can lead to injury to people and damage to equipment.

The furnace design ensures a relatively large reservoir of solidifiedbath (12) by suitably adjusting the ratios of the shaft width (22), theheight (21) of the shaft above the upper surface (10) and the width ofthe hearth (not shown) to the angle of repose (18) of the feed material.

If there is no feed material above the meniscus the whole of the metalbath will melt or become liquid and excessive superheating will takeplace with further heating. The present invention prevents this fromhappening by maintaining a suitable amount of feed material (16) in theshaft (6) and above the bridge (23) to eliminate the possibility ofmelting the reservoir of solidified metal (12), due to the assuredfeeding of material through the meniscus (10).

It is therefore apparent that the furnace can be operated withelectrical energy input suitable for any desired production rate, evenmaximum rate continuously, without the problem of superheating of themetal bath. This is achieved by maintaining a suitable amount of feedmaterial above the bridge and in the shaft. The amount is determined bythe volume of feed material necessary to ensure that the feed materialsupply frequency and batch size does not result in the material abovethe bridge and in the shaft to be depleted before it can be replenished.This, together with the reservoir of solidified metal (12) preventssuperheating. As far as the operator is concerned, superheating istherefore prevented by ensuring that the charging shaft (16) is keptfully charged with feed material (15, 16).

A second embodiment of an induction furnace according to the inventionis shown in FIGS. 3 and 4, in which like features are numbered the sameas in FIGS. 1 and 2.

FIG. 3 shows a perspective view of a furnace (1) according to the secondembodiment. As can be seen the furnace (1) has a feed hopper (6), a roof(5), sidewalls (4), a floor (3), and an induction heater (not shown) incommunication with the hearth (not shown) through a throat (7). Theouter shaft (19) around the inner shaft (not shown) allow the gassesformed during melting and by combustion of fuel to pass around the innershaft before being exhausted. The gasses are exhausted through an outlet(25) underneath the hopper (6). In this embodiment the inner shaftcomprises a plurality of feed tubes (24) that extend between the feedhopper (6) and the hearth (2).

In FIG. 4 the furnace (1) is shown with the outer shaft (19) and roof(5) removed. This shows the feed material (15) in the furnace (1).

The advantage that this arrangement offers is that fine, lowpermeability material can be preheated without the combustion gassescoming into contact with the material. This arrangement also contributesto feeding the feed material evenly to the furnace.

It will be understood that the above are only two examples of inductionfurnaces and a method of controlling them according to the invention,and that there are also other embodiments of the invention.

For example, the shaft can be designed to be vertically movable. Thisallows the height of the shaft above the meniscus to be changedaccording to changes in operating conditions and provides a furtherlevel of control over the process, by enabling the access of combustiongas to the shaft to be varied as a function of the feed materialcharacteristics.

It is also possible to operate induction furnaces used for theprocessing of other metals, such as copper and brass, according to thisinvention. For such other metals, specific furnace and shaft design willlikely be different from that of a steel induction furnace but theprinciple will be the same.

It is also possible to have heaters in the shaft sidewalls to furtherimprove the preheating of the feed material.

1. An induction furnace with a hearth having sidewalls and a floor, andan electrical induction heater in communication with the hearth throughthe floor by means of a throat to, in use, heat metal contained in thehearth to form a metal bath, characterised in that the furnace isprovided with means to control superheating of the metal bath bymaintaining regions of the metal bath adjacent the sidewalls at leastpartly solidified.
 2. A furnace as claimed in claim 1 in which thesuperheating control means includes means to substantially preventcharged feed material from contacting the sidewalls.
 3. A furnace asclaimed in claim 2 in which the furnace includes a central chargingshaft above the hearth.
 4. A furnace as claimed in claim 3 in which thefeed material is substantially prevented from contacting the sidewallsby charging the feed material through the shaft to the centre of thefurnace.
 5. A furnace as claimed in any one of claims 1 to 4 in which abridge of slag or metal, or a combination thereof, is formed over atmost part of the metal bath and the bridge at least partly supportscharged feed material.
 6. A furnace as claimed in claim 5 in which thefeed material penetrate the bridge at a determinable frequency and thefurnace is designed to operate at the frequency of bridge penetrationsthrough use of the parameters of liquid metal bath depth, shaft innercross sectional area, and the distance between the shaft opening intothe hearth and the bridge.
 7. A furnace as claimed in claim 6 in whichthe furnace is designed to have an amount of feed material supported bythe bridge that is adequate to collapse the bridge into the liquid metalbefore the heating of metal bath causes the entire metal bath to bemelted.
 8. A furnace as claimed in any one of claims 1 to 7 in which thesuperheat control means includes means to control the amount of slag inthe furnace.
 9. A furnace as claimed in claim 8 in which the furnaceincludes means to preheat the feed material.
 10. A furnace as claimed inclaim 9 in which the feed material is preheated in the hearth and theshaft.
 11. A furnace as claimed in claim 9 or 10 in which the feedmaterial is preheated through the combustion of fuel.
 12. A furnace asclaimed in anyone of claims 9 to 11 in which the furnace includes feedtubes extending between the shaft and the hearth, feeding of feedmaterial is done through the feed tubes, and preheating is at leastpartially done in the feed tubes.
 13. A method of controllingsuperheating of a metal bath of an induction heated furnace with ahearth having sidewalls and a floor, and an electrical induction heaterin communication with the hearth through the floor by means of a throat,by maintaining the metal bath at least partly solidified.
 14. A methodas claimed in claim 13 in which the metal bath is maintained at leastpartly solidified in regions of the metal bath adjacent the sidewalls.15. A method as claimed in claim 14 in which the metal bath ismaintained at least partly solidified by allowing a bridge of solidifiedslag or metal, or a combination thereof, to form over the metal bath andpreventing the bridge from contacting the sidewalls.
 16. A method asclaimed in claim 15 in which the bridge is prevented from contacting thesidewalls by preventing feed material from contacting the sidewalls. 17.A method as claimed in claim 16 in which feed material is prevented fromcontacting the sidewalls by charging feed material to the centre of thefurnace through a charge shaft located centrally above the hearth.
 18. Amethod of operating an induction heated furnace with a hearth havingsidewalls and a floor, and an electrical induction heater incommunication with the hearth through the floor by means of a throat,including the steps of a. providing a metal bath in the hearth, themetal bath including regions of at least partly solidified metaladjacent the sidewalls to form a solidification front between the atleast partly solidified metal and liquid metal in the bath; b. chargingfeed material into the furnace; c. allowing a bridge of solidified metalor slag, or a combination thereof, to form over at most part of themetal bath, the bridge supporting at least part of the charged feedmaterial; d. heating the metal bath to cause the solidification front tomove into the partly solidified metal and the bridge to be weakened tothe point where the weight of the feed material supported by the bridgecauses the feed material to collapse the bridge into the liquid metaland the feed material to penetrate into the liquid metal; e. allowingthe penetrated feed material or collapsed bridge to lower thetemperature of the liquid metal to cause the solidification front tomove closer to the throat; f. allowing the bridge to be reformed tosupport subsequently charged feed material; and g. tapping molten metalfrom the furnace.
 19. A method as claimed in claim 18 in which step 18.bincludes charging feed material substantially to the centre of thefurnace.
 20. A method as claimed in claim 19 in which the feed materialis charged through a central charging shaft above the hearth.
 21. Amethod as claimed in anyone of claims 18 to 20 which include the step ofpreheating the feed material.
 22. A method as claimed in claim 21 inwhich the feed material is preheated in the hearth and the shaft.
 23. Amethod as claimed in claim 21 or 22 in which the feed material ispreheated through the combustion of fuel.
 24. A method as claimed in anyone of claims 18 to 22 which include the step of removing slag from thefurnace.